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The microstructure of machined superficial layer significantly affects the mechanical properties and service life of mechanical components. Herein, to obtain a machined surface with excellent properties, a comprehensive application of theoretical calculations and test analysis research methods was conducted to investigate the influence mechanism of machining on performance and quality of machined superficial layer of high-strength alloy steel during machining, and reveal the intrinsic correlation between microstructure evolution, quality and performance of machined superficial layer. The results indicate that the machining can achieve gradient microstructure on the machined surface, which can improve the performance of machined surface greatly. From the surface to the inside region, the layers are respectively the recrystallized layer where compact nano-sized equiaxed grains are located, the rheological layer with clusters of high-density sub-grain structures, and the distorted layer with residual distorted grains. The maximum strain of superficial layer in the machined state is 2.81 times that of the original state. The grains are evidently refined and the grain size is reduced by 49.03%. The dislocation density is enhanced by 100.69%, while the number of small-angle grain boundaries (SAGB) and other substructures increase sharply, being 5.26 times that in the original state. By performance testing, the machining substantially enhances the hardness of the superficial layer of high-strength alloy steel with certain rise in toughness simultaneously, which is exactly the result of the combined effect of fine-grain strengthening and dislocation strengthening caused by the machining process above.